This invention relates to a method of operating a refrigerant cycle with a failed suction pressure sensor to ensure that undesirably low suction pressures do not occur.
Moderate refrigerant cycles are typically controlled by microprocessor control algorithms. A number of variables are taken in as feedback, and utilized to determine optimum conditions for the various components in the refrigerant cycle. One type of refrigerant cycle which has had a good deal of recent development of such controls is a refrigerant cycle for large refrigerated transport vehicles. These transport vehicles are utilized to transport frozen or perishable items, and typically food stuffs.
The refrigeration of such containers is particularly challenging when perishable items are being stored in the containers. Perishable items are not kept frozen, but must be kept within a very tight temperature band. Such systems attempt to control the temperature by controlling the various components in the refrigeration cycle. Among the components which are typically controlled are the refrigerant compressor and a suction modulation valve (SMV).
During this control, it is possible that the suction pressure can drop to undesirably low values at the compressor. One problem that can occur if the suction pressure is undesirably low is that there could be Corona discharge across high voltage terminals in the motor which drives the compressor. This is undesirable, but will typically not occur if the suction pressure is above 1.0 psia.
Thus, the prior art has incorporated controls including a suction pressure sensor that ensures the suction pressure does not fall below this amount. The control monitors the suction pressure and if the suction pressure went below a predetermined amount approaching 1.0, then the control for the system takes steps to ensure the suction pressure does not continue to drop.
If the suction pressure sensor fails, the prior art system was turned off. Users of the refrigerant equipment developed methods for replacing the suction pressure sensor input to the controller. Thus, a xe2x80x9cfalsexe2x80x9d signal would be sent to the controller to replace the missing signal from the failed sensor. Of course, such a method of replacing a valid signal with a false signal eliminates the protection provided by the control algorithm.
The present invention is directed to a method that will allow continued operation of the system even when the suction pressure sensor fails.
In the disclosed embodiment of this invention, a controller for a refrigerant cycle continues to operate essentially as in the prior if a valid suction pressure signal is received. However, in a preferred embodiment, if a valid pressure sensor signal is not received, then the system moves into a mode wherein a minimum open percentage for an SMV is maintained. Applicant has determined that the suction pressure varies with the percentage that the SMV is open. For a given ambient temperature, a minimum SMV open percentage can be defined to ensure that the suction pressure will not drop below a predetermined amount.
Most preferably, this minimum open percentage is set to provide a large margin of error such that any unpredicted variables will still not result in the suction pressure dropping below the 1.0 psia number mentioned above.
This invention thus sets the SMV percentage open number as a minimum in a situation where the suction pressure sensor has failed, and does not close the SMV even if the control algorithm would suggest further closing of the SMV beyond this number.
Most preferably this system is incorporated into a refrigerant cycle for a refrigerated container.